5. Functions and actions

Both pure functions and actions use the keyword def; the ! suffix on the name decides which it is. These are the first two of Hanki's three layers (§1, The three layers); the third - actors (§15) - is built from actions.

A pure function has no side effects and may not call any action.

def sum(x: i32, y: i32) -> i32
  x + y
end

An action may have side effects. Action names always end in !. The function type carries an optional effect annotation […] after the return type.

use io

def print_hello!(name: string) -> () [io]
  io.print!("Hello, #{name}")
end

Rules:

• A def whose name does not end in ! is a pure function. No effect annotation allowed.
• A def whose name ends in ! is an action. The effect annotation […] is optional (defaults to the empty set, but the ! is what authorises any effects at all).
• A pure def cannot call any action. The ! suffix at the call site is a visible reminder.
• An action can call pure functions and other actions whose effects are a subset of its own.
• Types are mandatory on all parameters and the return type. No silent defaults.